8 Br. Huggins [Jan. 20' 



We have now advanced to the extreme boundary of the solid ground 

 of our knowledge of comets. Before us lies the enchanted region of 

 speculation. Without being too venturesome, we may well consider 

 a few points which may explain more in detail some of the phenomena 

 of comets. Of whatever nature we may regard the tremendous changes 

 which take place in them to be, we must certainly look for the primary 

 disturbing couse to the sun. Is the solar heat sufficient to account 

 directly for the self-light of comets, or does it act the part of a 

 trigger, setting free chemical or electrical forces ? On this point of 

 the sufficiency of the solar radiation we must not look to the few cases 

 of excej)tionally close approach to the sun, but to the more average 

 distance of comets at perihelion. Professor Stokes has suggested 

 that some results obtained by Mr. Crookes may throw light upon this 

 question. He concluded from his experiments that in such vacua as 

 exist in planetary space the loss of heat, which in such cases would 

 take place only by radiation, would be exceedingly small.* In this 

 way the heat received from the sun by the comet would accumu- 

 late, and we should get a much higher temperature than would other- 

 wise be j)ossiblc. In this connection may be mentioned the remark- 

 able persistence of the bright trains of meteors in the cold upper air, 

 which sometimes remain visible for three-quarters of an hour before 

 the light fades out by the gradual dissipation of the energy. 



I need hardly say that the enormous tails of bright comets, many 

 millions of miles in length, cannot be considered as one and the same 

 material object, brandished round like a great flaming sword, as the 

 comet moves about the sun. It is but little less difficult to suppose that 

 the cometary mass is of so large an extent as to include all the sjiace 

 successively occupied by the sweep of the tail at perihelion. On the 

 material theory we seem to be shut up to the view that the tail is con- 

 stantly renewed and reformed, either by matter streaming from the 

 nucleus or in some other way. But this view involves velocities far 

 greater than the force of gravitation can account for. Let us consider 

 the order of the jjhenomena. Under the sun's influence, luminous jets 

 issue from the matter of the nucleus on the side exposed to the sun's 

 heat. These are almost immediately arrested in their motion sun- 

 wards, and form a luminous cap ; the matter of this cap then appears 

 to stream out into the tail, as if by a violent wind setting against it. 

 Now, one hypothesis supposes these appearances to correspond to the 

 real state of things in the comet, and that there exists a repulsive 

 force of some kind acting between the sun and the gaseous matter, 

 after it has been emitted by the nucleus. On this hypothesis the 

 forms of the tails of comets, which are usually curved, and denser on 

 the convex side, admit of explanation. Each particle of matter of the 

 tail must be moving in a curved course, under the influence of the 

 motion it originally possessed, combined with that of this hypothetical 

 repulsive force. But in the form which the tail assumes for us we 



* I 



Proceedings R. S.' 1880, p. 248. 



